We have raised an antiserum which is labelling specifically neurofibrillary tangles of Alzheimer's disease. We show here that anti-tangles detects on immunoblots exclusively Tau, a set of proteins that promote microtubule assembly. Conversely, an anti Tau detects histologically Alzheimer tangles. Accordingly, neurofibrillary tangles are very likely composed of aggregated Tau proteins, due to an anomaly of microtubular protein synthesis or to a general suffering of certain pyramidal neurons leading to an elective aggregation of Tau.